Estimates of battery life vary and I've seen figures of between 500 and 2000 charge cycles quoted. Current thinking seems to be that this is full charges and that the number increases pro rata for partial charging. To prolong life, there are two things you can do. The first, and most important, is not to let the battery go flat - they don't like it and, in certain circumstances, you can brick the phone. The other is not to charge to 100% as this can degrade the electrodes. Keeping between 30 and 90% seems to be the sweet spot, although I don't think any of us actually does that!
Battery life is coming to the fore as people aren't upgrading their phones on an annual/biannual basis. As technology developed fast, this had a benefit as software made increasing demands on processors and storage, but changes between models are becoming smaller and smaller and OS updates confined to cosmetics and security patches. As a result, phones last longer and battery life is becoming an issue. Apple's speed-throttlling has brought this to the forefront (and I can't help wondering whether that might have been deliberate). Maybe we will see a return to user-replaceable batteries, though this will need to be demand-driven as manufacturers will still be looking for ways to persuade us to buy the latest models (assuming the annual product-line refresh schedule continues). A user-replaceable battery will mean a move away from super-slim devices, so that's a consideration, too.
